Registering x64 unmanaged/C++ .dll

I'm developing an ArcObjects extension that was recently ported to 64-bit. I'd need to take advantage of 64-bit geoprocessing, however I can't figure out how to properly register the .dll. ESRIRegAsm.exe doesn't seem to work with x64 .dll's

I found this knowledge base article, but it only refers to managed code (AnyCPU config). I need the instructions for an unmanaged .dll using x64 config.

There's also this reference in the ArcGIS help, but once again, it doesn't explain how to register unmanaged .dll. "For tool developers working with unmanaged code, C++, for example, you will need to build both 32-bit and 64-bit versions of your tool, then ship and register both for proper use." So, the question is, how??